In a communication carrier network, communications to a particular regional network are managed by a particular station, and a call control server is responsible for executing tasks such as call control for the station. Because a call control server may become congested by processing load of incoming and outgoing calls to and from the terminals in the particular region managed by the call control server, congestion control using central processing unit (CPU) utilization is run on the call control server.
Specifically, the call control server monitors the CPU utilization at a constant cycle. If the CPU utilization exceeds a threshold N times in a row (where N is any natural number), the call control server considers that congestion has occurred, and incrementally suppresses the amount of call control handled by the call control server.
For example, when the average CPU utilization exceeds 75 percent, the call control server considers that moderate congestion has occurred, and suppresses 30 percent of the entire traffic. When the average CPU utilization exceeds 85 percent, the call control server heavy congestion has occurred, and suppresses 50 percent of the entire traffic. When the average CPU utilization exceeds 95 percent, the call control server considers that system congestion has occurred, and suppresses the entire traffic.
The call control server uses software control to reject the calls determined to be targets of suppression. Once the average CPU utilization drops below the threshold the traffic suppression is executed, the call control server releases the congestion control, and starts the call control again. Related-art examples are disclosed in Japanese Laid-open Patent Publication No. 2009-296186 and Japanese Laid-open Patent Publication No. 2010-74310.
With such a technology, however, congestion may prohibit establishment of calls even when some transmission line resources, i.e., some transmission circuit resources are available.
For example, when the call control server receives a large number of incoming calls and outgoing calls repeatedly in a short time period, after a disaster occurs, for example, the CPU becomes congested before the accepted calls are transitioned to the calling or busy status, for example. Because the CPU utilization is high, the call control server rejects acceptance of calls, despite transmission lines for establishing the calls might be available. Such available transmission line resources may therefore remain unused.